Particle beam generator

ABSTRACT

The source of electrons is a nanotip in a vacuum as used in near field microscopy. The source of ions is a similar nanotip in vacuum supplied with liquid metal (gallium) as in a liquid-metal ion source. Electrons or ions from this nanometre-sized tip are extracted by centralising the tip over an aperture plate and applying a suitable voltage to the tip. The electrons (ions) pass through this plate and are accelerated up to several keV using a nanoscale/microscale accelerating column before being focussed using further microscale (or nanoscale) cylindrical lenses. The final element is an aberration corrected miniature (or sub-miniature) einzel lens which can focus the beam at several millimetres from the end of the instrument.

The present invention relates to the generation of focussed particlebeams (in vacuum) and particularly to electron and ion beams for use in,for example, microscopy, such as, for example, scanning electronmicroscopy (SEM), and nanotechnology, such as, for example, innanolithography in the production of nanostructures and nanostructuredsurfaces by direct write techniques such as ion beam milling(sputtering), for the case of focussed ion beams (FIB), and surfacemodification methods, such as polymerisation or oxidation, for electronbeams.

Known particle beam generators, for use in microscopy and lithography,generally comprise a particle source, operable to provide illumination.This is usually a sharp tip from which particles, such as electrons orions, are extracted by application of a relatively strong electricfield, that is, a field emission source. Alternatively, a heatedfilament (tungsten hairpin) may be used as a source. However, a fieldemission source is advantageously used in applications where relativelyhigh spatial resolution imaging is desirable. The source usually alsocomprises a voltage driven particle accelerator to increase the particlebeam energy to a desired level. A focusing system, which may comprise,for example, magnetic lenses, is controlled to focus the acceleratedparticles at a cross over point to form a beam spot on a surface of amaterial. In the case of microscopy the material would be the sampleunder investigation.

For microscopy, such as for the SEM, a set of coils are used tofacilitate scanning of the beam over the sample. The sample is mountedon a stage disposed below the field of an objective lens. Thespecification and properties of the objective lens and the distancebetween the objective lens and the sample, that is, the workingdistance, dictate the resolution limitations of the microscope. Adetection system, operable to detect secondary and backscatteredelectrons, is usually disposed below the objective lens. Known nearfield microscopy instruments are disadvantaged in that the position ofthe detection system dictates the working distance of the microscope andtherefore prevents short working distances being advantageouslyutilised, thereby limiting the optimum resolution achievable by themicroscope.

The size of the final beam spot and the amount of beam current in thisfocussed spot determine the performance of these instruments. Formicroscopy the beam spot size is the effective spatial resolution of theinstrument and for nanolithography it determines the minimum sizefeature which can be made. The current state of the art for commercialparticle beam generators is 1 nm for electrons and 30 nm for metallicion beams.

It is desirable in microscopy and lithography for there to be a particlebeam generator operable to provide a beam having a greater optimumresolution than is currently available. More particularly, it isdesirable for there to be a particle beam generator suitable for use innanoscale analysis of samples in microscopy and in nanolithography. Suchresolution would provide atomic identification at a significant depth offield and provide surface analysis at nano-scale dimensions.

Furthermore, the relatively long working distance, as required in knowninstruments, is also a disadvantage in that it necessitates applicationof a relatively high particle acceleration voltage to achieve optimumresolution at that distance. However, a higher particle accelerationvoltage increases the energy of the particle beam, which, at an upperthreshold, may cause undesirable increases in inelastic scatteringwithin the material structure thereby causing radiation damage to thematerial being examined. In microscopy, a relatively low energy beamhaving a relatively high resolution provides the possibility of reducedinelastic scattering of the material electrons, relative to knownapparatus, to such an extent to enable complex molecule structures to beanalysed.

Therefore, it is desirable in microscopy and lithography for there to bea particle beam generator operable to provide a beam comprisingparticles accelerated using a lower voltage than is currently available.

Although, the SEM is specifically mentioned above as an example ofmicroscopy, the reader will appreciate that other so-called near-fieldmicroscopy instruments exist such as, for example, the ScanningTunnelling Microscope (STM) and Atomic Force Microscope (AFM).

It is therefore desirable for there to be a particle beam generatoroperable to provide a beam, suitable for use in nano-scale dimensionapplications, comprising particles accelerated using a relatively lowvoltage, which is suitable for use with known near-field microscopeinstruments.

Furthermore, such near-field microscope and lithography instruments areexpensive and it is desirable to increase resolution and depth of fieldand/or decrease the accelerating voltage without having to replace thewhole of the instrument.

Known microscopy and lithography instruments are also disadvantagedbecause they are vulnerable to vibration which can effect the operationthereof and therefore it is desirable for there to be microscopy andlithography instruments which are less susceptible to vibration.

The present invention provides a particle beam generator, suitable foruse in nanometre technologies, comprising an extractor plate, having anextractor aperture, disposed adjacent a particle source and operable toextract particles from such a source into the extractor aperture to forma particle beam, particle accelerating means operable to accelerate theextracted particles to increase the energy of the beam, and collimatingmeans operable to collimate the particle beam, characterised in that atleast one of the extractor aperture and the accelerating means inhibitslateral expansion of the particle beam to provide a near parallelparticle beam having a diameter less than 100 nm.

The particle beam generator may further comprise focussing meansoperable to provide, from the laterally inhibited particle beam, afocussed particle beam having a diameter less than 1 nm.

The present invention utilises scale invariance of particle trajectoriesin electric fields. The absolute size of the beam spot may be related tothe overall size (in particular the focal length of focussing lenses) ofthe active elements of the instrument. These elements (in order fromsource to final beam spot) are a particle source and accelerating means,which may act, individually or in combination, to inhibit lateralexpansion of the particle beam. Such particle beam generators may besub-miniature and contain micro-machined focussing and acceleratingmeans which inhibit expansion of the beam. Therefore the resolution maybe kept much smaller than in larger instruments. Thus a design made atthe scale of 100 mm may have beam spot sizes at least substantially 100times larger than a micro-machine with maximum sizes of millimetres.Although a larger instrument will allow use of higher voltages and thusaccelerate the beam to higher energies which may result in smaller beamspot sizes, even when this is taken into account, the beam spot size ofsub-miniature designs may be at least substantially 10 times smallerthan a substantially identical larger instrument.

The diameter of the extractor aperture may substantially be between 5 nmand 500 nm. More preferably, the diameter of the extractor aperture maysubstantially be between 5 nm and 100 nm.

The particle accelerating means may comprise a plurality of acceleratorplates arranged in a stack and electrically isolated from each other.Each of the accelerator plates may comprise an aperture adapted to sharea common longitudinal axis with the extractor aperture to thereby forman extended accelerating aperture. On application of a voltage betweenthe extractor plate and a first acceleration plate and between each pairof successive adjacent acceleration plates arranged in the columnthereafter, extracted particles may be accelerated through theaccelerating aperture and thereby increase the energy of the beam ofwhich they are constituent parts.

Alternatively, the extractor plate may be a first conductor which isseparated form a second conductor by at least one of a resistive andinsulator material and the accelerating means may comprise anaccelerating aperture which extends from the extractor aperture throughthe material and through the second conductor, wherein extractedparticles are accelerated through the acceleration aperture onapplication of a differential voltage between the first and secondconductors. Preferably, the resistance of the material is substantiallybetween 1 kΩ and infinity. The material is preferably a semiconductormaterial and advantageously doped Silicon.

Advantageously the diameter of the accelerating aperture issubstantially between 10 nm and 1000 μm. The collimating means may beintegrally formed with the accelerating aperture and advantageouslycomprises a conical formed in the wall thereof. The conical shape beingsuch that the diameter thereof increases in the direction of theaccelerated beam.

Alternatively, or additionally, the collimating means may comprise atleast one collimating aperture having a lesser diameter relative to theaccelerating aperture and may be disposed on the longitudinal axisthereof.

The particle beam generator advantageously comprises a particle sourceintegrated therewith, which is preferably a field emission source.

The particle beam generator may be adapted for use with an electronparticle source or, alternatively, may be adapted for use with an ionparticle source.

The particle beam generator may form part of a near field microscope andmay be mounted on a microchip.

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram, through section A—A, of a particle beamgenerator according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram of an example of first stage focussingmeans suitable for use with the particle beam generator of FIG. 1;

FIG. 3 is a schematic diagram of an example of second stage focussingmeans suitable for use with the particle beam generator of FIG. 1 andthe first stage focussing means of FIG. 2.

FIG. 4 is a schematic diagram of the particle beam generator of FIG. 1and the first and second stage focussing means of FIGS. 2 and 3,respectively;

FIG. 5 is a schematic diagram of the particle beam generator accordingto a second embodiment of the present invention;

FIG. 6 is a schematic diagram of a particle beam generator according toa third embodiment of the present invention;

FIG. 7 is a schematic diagram, in section, of a particle beam generatoraccording to a fourth embodiment of the present invention, also showingcollimating means;

FIG. 8 is a schematic diagram, in section, of a microscope elementcomprising a particle beam generator according to the present invention;and,

FIG. 9 is a schematic diagram of further collimating means.

Referring to FIG. 1, a particle beam generator 10 comprises an extractorplate 12, having an extractor aperture 13, positioned using apiezo-electric control system (not shown) so that it is locatedcentrally with respect to a particle source 14. The particle source is anear field nanotip source. The nanotip is a standard SEM tip with aradius of around 8 nm. The dotted circular line indicates that theextractor plate can be laterally much larger than indicated. Theaccelerator column 16 consists of a series of acceleration plates 18stacked to form a column, each plate having an aperture adapted suchthat when the plates are stacked they share a common longitudinal axiswith each other and with the extractor aperture 13, to form an extendedacceleration aperture 20. Each plate 18 is electrically isolated fromeach other and can be supplied with its own voltage. The voltages on theplates 18 and the nanotip 14 are shown on the right hand side for thecase of accelerating electrons or negative ions. For this case V isalways negative and the final energy of the electrons from the column isVT in electron volts (eV). ΔV is the voltage difference between eachplate in the column and VO is the difference in voltage between the tipand the extractor plate. (The largest negative voltage is on the tip andthe voltages increase moving down the column to the final plate at zerovoltage.) The particle beam generator 10 may be adapted for use withnear field microscope apparatus and may be designed for operation in theenergy range from 300–1000 eV. The voltages and separations of theelectrodes are adjusted so that the nanotip emits electrons and thefield in the accelerating aperture is that required to produce aslightly converging beam. Electron trajectories are schematicallyindicated by the dot/dashed lines 22 with the electrons travelling fromthe top to the bottom of the diagram. These trajectories indicate theoverall beam profile which is defined by the envelope which contains themajority of the electrons which are emitted from the tip and passthrough the accelerator column.

Referring to FIG. 2, first stage focussing means 24 are shown comprisinga first micro-scale lens system. This micro-scale lens system isdisposed to collect and focus the particle beam from the acceleratoraperture 20. FIG. 2 shows the focussing effect on the beam profile 26.This lens is an aberration corrected cylindrical einzel lens consistingof three cylindrical elements 28, 30 and 32. The outer two elements 28and 32 are at earth potential and the central element is supplied with avoltage sufficient to focus the electrons at the required position.(Either polarity voltage can be applied but the aberrations are thesmallest for a positive voltage, when used to focus electrons, and anegative voltage when used to focus positively charged ions.) Anapproximate scale of this particular micro-lens is shown at the top ofthe figure. As an example, in the diagram the beam is focused at asample holder 34 a which can be moved laterally to scan the sample andalong the beam axis to adjust the focus. The aberrations in this lensare corrected by adjusting the relative dimensions marked a, b, l and ton the sections of FIGS. 1 and 2.

Referring to FIG. 3, a second stage focussing means 36 are shown,comprising a miniature einzel lens consisting of three cylindricalelements 38, 40 and 42. It is essentially the same as the previous lensexcept that it is approximately a thousand times larger and focuses thebeam 26 at a point several millimetres from the end of the instrumentwhere a sample holder 34 b is positioned. As previously the scanning isachieved by moving the sample laterally using a piezo-electric controlsystem. Also the holder 34 b can be moved along the axis to place thesample at the exact focus. Because the focal length is millimetres it isnow possible to include an electron detector 44 in the space above thetarget. This is used to detect and measure the back-scattered electronsso that scanning images can be obtained. It is most important that thislens is corrected as well as possible for aberrations. In addition torelative adjustments of the dimension a, b, l and t, the curvature ofthe inner surface 46 shown by a dot/dash line can be also optimallyshaped.

In use, the extractor plate is disposed in close proximity to a particlesource and a voltage is applied between the plate and the source causingelectrons to be emitted directly from the tip by the process of fieldemission. A similar process can produce an ion beam if liquid metal issupplied to the tip as in focussed ion beam sources. The brightness ofthese electron/ion beams is extremely large and they can be thereforefocussed to small spots. To use this beam and inhibit it from laterallyexpanding an extractor plate having a nanoscale extractor aperture isused, followed by a high electric field region on the side of the plateopposite to the nanotip. Thus the electrons/ions can be successfullyextracted from the nanotip source and pass through the extractoraperture, which can be centred on the nanotip source by moving theextractor plate using piezo-electric translation devices as commonlyemployed in near field spectroscopy. The electric field on the oppositeside of the extractor plate is made to be similar to that on the sidefacing the nanotip source accelerating the electron/ions and at the sametime producing a weak focussing effect. The particle beam size followingthis aperture is essentially determined by the aperture size andcalculations show that most of the electrons or ions emitted from ananotip source can be formed into this particle beam if the aperture isaround 30 nm in size.

This design of source is different to that conventional employed in thatit uses a nanoscale aperture positioned close to the tip preferably lessthan a few hundred nanometres away. Thus electrons can be extractedthrough a minute aperture and can therefore be subsequently confined tosmall dimensions close to the axis of the following lenses. Also itmeans that much smaller voltages are needed to generate field emissionfrom the tip. By using a nanoscale/micro-scale accelerating columnhaving an accelerating aperture extending from the extractor aperture itis possible to generate an approximately equal electric field on eitherside of the extractor plate so that it is possible for the extractorplate to act as a weak lens. This is in addition to its (theaccelerating aperture) function of accelerating the electrons/ions. Thusthe beam is not allowed to expand significantly in its progress throughthe instrument which, because of the small size of the beam, limits theunwanted effects of lens aberrations, and allows the use of cylindricalfocussing lenses (both electrostatic and magnetic) with apertures in therange from 1–1000 μm which considerably benefit from the overalldecrease in scale of the instrument.

This allows the use of focussing lenses with micro-scale (sub-miniature)and millimetre focal lengths. Since these focal lengths are considerablysmaller than conventional electron microscopes it is possible to focusthe beam down to much smaller dimensions with fewer corrections for lensaberrations.

These miniature and sub-miniature designs are for operation as standalone instruments for electron/ion energies up to a maximum of a few keVbut they may also be employed as the first stages of a largerconventional high energy electron/ion beam system working up to andbeyond 100 keV.

The beam from the source accelerator column then passes through amicro-scale cylindrical einzel lens positioned at a distance such thatthe beam from the end of the accelerator column has not expandedsignificantly before it reaches this lens. It is then possible to focusthe beam, using this lens, down to diameters below one nanometre atseveral microns distance from the final lens element. In order to getthe smallest focal spot this element is corrected for aberrations byadjusting its geometry as described later.

Although it is possible to use this focussed beam spot directly for SEMor FIB techniques it is more practical if the beam is then passedthrough a miniature, or sub-miniature, einzel lens with typical aperturediameters from a few hundred to several thousand microns. This lens ispositioned at an optimal distance from the first micro lens such that itis possible to obtain the smallest beam spot at distances of millimetresfrom the end of the last lens element of the lens. Such an arrangementis much more practical and allows for the insertion of electrondetectors normally needed for SEM.

Although the beam size through this last lens can be less than a fewmicrons it is still necessary to correct for aberrations (mainlyspherical) to achieve the best performance. This is done by altering itsgeometry as detailed later. Focussed beam spot sizes significantlysmaller than 1 nm can be obtained if this lens is properly corrected.

It will be appreciated by a person skilled in the art that other firstand second stage focussing means may be utilised which may be equallyapplicable to the working of the invention.

Referring to FIG. 4, a complete system is shown comprising twofour-element, cylindrical, einzel lenses, one microscale, labelled, B,and one miniature, labelled, C. These lenses are situated downstream ofthe electron/ion source labelled, A. By having many variables in thegeometry they can be made to have very low aberrations. This is somewhatanalogous to multi-element optical lenses in cameras and opticalinstruments. The final design for these beam elements depends on theelectron/beam energy, divergence and size as it enters the lens. Theparameters which can be varied are:

-   1) The number of elements-   2) The thickness of each electrode-   3) The spacing between the electrodes-   4) The aperture sizes in each electrode-   5) The shape of the edges on the lenses as shown in the previous    application-   6) The voltages applied to each electrode

We have been able, by suitable variations in these parameters to almostcompletely eliminate spherical aberration from our system.

FIG. 4 shows the geometry of a four-element lens with electrodeslabelled 48, 50, 52 and 54 with voltages V1, V2, V3 and V4 respectively.The beam and its direction are labelled 56. A first analysis position,58, is a focus distance, f1 from the end of the microscale lens.Scanning of the beam is achieved by moving the sample using piezos as isusual in scanning tunnelling microscopy. This sample position can beremoved and the beam made to travel through the second miniature lens soas to come to a focus at a distance, f2, from the end of the secondlens. At this point there is a piezo driven sample holder, 60. Althoughthis second miniature lens is shown as having the same geometry as thefirst lens this need not necessarily be the case. Again the exactgeometry (aperture sizes etc) will depend on the beam properties as itpasses through this lens. Typical aperture sizes are around 5 μm for themicroscale lens and 5 nm for the miniature lens but these can be variedover a wide range.

A further embodiment of the present invention is shown in FIG. 5,wherein a particle beam generator is a micro-chip 100 comprising one ormore nanocolumns 162 which produce a narrow (<50nm) on-axis beam. Ananotip 114 is at the end of a microstructure which is attached to thevertical cantilever (not shown) and positioned centrally and greaterthan 10 nm from the first aperture 113 of the nanocolumn 162. Thenanocolumn 162 can be in one or more parts as shown and defines an axialbeam of lateral dimensions less than 50 nm. A typical nanocolumn 162 isshown in figure 5 b and is made of a thin multi-layer film consisting ofalternate metal (conducting) layers, 118, interspaced with insulatinglayers 119 through which a circular aperture 113 is made by lithographyor using a focussed ion beam (FIB) ‘milling machine’. The total lengthof the nanocolumn(s) may be up to 2μm and is sufficient to accuratelydetermine the (on-axis) direction and phase space emmitance of the beam.The nanotip 114 is positioned above the aperture as shown and a voltagediferrential is applied between the tip 114 and the nanocolumnelectrodes 162. The beam defined by the nanocolumn has an axis 164 whichis concentric with the multi-element, microscale, einzel lens. This lensconsists of metal (conducting) electrodes 166 interspaced withinsulators 168. The assembly shown consists of four metal electrodesinterspaced with insulators and is positioned at distances of only a fewmicrons from the nanocolumn from which it is separated by an insulatingfilm with an aperture of the same dimension as the microlens. Suitableaperture diameters for this lens are given in the previous application.Increasing the number of metal conducting electrodes in the stack canreduce aberrations in this lens.

FIG. 6 shows one of the ways of constructing the microscope so that itis possible for the microlens to focus the beam at a point less than 50μm from the end of the instrument. This condition is necessary if thebeam is to have a lateral size less than 1nm and approaching 1 Å. (Thisbeam spot essentially determines the resolution of the instrument.) Anapplication of this embodiment of the present invention is shown in nearfield microscopy in FIG. 6b and consists of the ‘chip’ or body 100rigidly attached to a horizontal cantilever arm 170, of a near fieldmicroscope, which can be positioned using standard techniques ofnanopositioning. A vertical cantilever above this holds the nanotip 114and this can be moved vertically and scanned in the horizontal plane.The sample is mounted on a special retainer 172 which has a smallsurface area for attaching the sample. (This atomic resolutionarrangement can only accommodate small area sample; for larger areas thefocal length of the microlens is increased and the resolution degrade toaround 1nm.) A further vertical cantilever 174 below the microscope bodyholds the sample retainer and provides a means of positioning the sampleat the correct vertical distance as well as scanning in the horizontalplane.

The details of the body or chip 100 are shown in FIG. 6 a A series ofsteps are produced by lithography, or micromachined with laser beams, inone edge of the chip. The bottom step is only a few μms thick and widerthan about 20 μm. On this step 176 are formed a multi-layer assembly 178which is essentially the body of the particle beam generator as shown inFIG. 6 a. The multi-layer is grown by atomic deposition in two stages.First the layers corresponding to the electrostatic lens are producedand a hole 113 corresponding to the lens aperture is fabricated in thelayers by lithography near to the edge of the step corresponding to theletter A in the diagram. (Many holes can be produced in one lithographicprocedure and each can be a separate microscope.) The top layer iscovered with a nanometre thick film of gold or carbon and the successivemulti-layer are then grown (by atomic deposition) corresponding to thelayer of the nanocolumn. The layers are produce using a horizontal maskwhich allows each separate layer to terminate at a different positionalong the step. This provides an essential method of attachingelectrical contacts to the electrodes in the microscope as illustratedby the exposed area 180. Finally the nanocolumn hole is drilled throughthe top multi-layer on axis with the electrostatic lens using a focussedion beam. (This can also be made by state of the art e-beam lithographyand dry etching techniques.)

In operation the nanotip 114 is centred on the aperture and the voltageson the lens adjusted to focus the beam, with axis 164 onto the sample atfocal point 182. The thickness of the support step 176 and the focallength are arranged so that there is a sufficiently large enough gap forthe backscattered electrons to be recorded with the channeltron, 184.

Two further adaptations are possible to allow the microscope to becontained in a single chip. Firstly the nanotip and a microscopiccantilever can be produced in the body of the chip. (This would probablybe a horizontal cantilever.) Secondly the detector can be fabricatedinto the base of the chip. For this purpose it is probably better to usea semiconducting avalanche type detector for the electrons. It is evenpossible to consider incorporating the mechanisms to scan the samplewithin the base of the chip to make the ultimate SEM on a chip.

In use, typical dimensions and voltages are as mentioned above for theprevious embodiment of the present invention. A suitable arrangement isfor the nanotip 114 to be positioned using a vertical cantileverarrangement as used in scanning tunnelling microscopy (STM). Howeverrecent advances in lithography make it possible to incorporate thiscantilever into the microscope itself. The nanocolumn consists of amultilayer of conducting (metal or silicon) thin films separated byinsulating layers through which a circular hole of the requirednanoscale size (typically less than 50 nm) is fabricated. This is madeeither by lithographic techniques or by drilling using a focussed ionbeam. A microscale multi-element einzel lens is positioned below thenanocolumn and concentric with it. This can be made as a separate partand can be independently positioned with respect to the nanocolumn usingstandard micropositioning sytems. In a simpler form the microlens ismade into the same multilayer structure as the nanocolumn and the wholeforms the basic element of the microscope. Fabrication of the lens canbe made by a variety of techniques including lithography and laser beammachining.

The microscope body is held on a vertical (cantlilever) arm which can bepositioned both vertically and laterally. The nanoprobe is centred onthe nanocolumn aperture by a servo-mechanism which uses the current in aquadrant metal thin film which forms the first electrode in thenanocolumn. Below the microscope body is a sample stage on the end of avertical cantilever. The electron beam is focussed onto the sample andcan be moved across the sample by moving the end of the cantilever usingstandard nanopositioning techniques as used in STM, for example usingpiezo-electric mechanisms. If the backscattered electrons are detectedusing an electron detector (chaneltron) then an image of the atomicstructure of the surface can be made. In the ultimate design theelectron detector can be incorporated into the base of the microscopechip.

The design for a focussed ion beam system suitable for machining surfacestructures below 10 nm can be adapted from the previous description byfeeding liquid gallium to the nanotip and reversing the polarity of thevoltages. In this arrangement the geometry is inverted so that thenanotip is positioned below the nanocolumn. (This is to prevent liquidgallium from contaminating the microscope.)

A further embodiment of the present invention is shown in FIGS. 7 and 8,wherein a particle beam generator 200 comprises a pair of thin filmmetallic layers, 212 and 213 separated by a semiconductor material 284.Each of the metallic layers comprises collimating apertures 286 and 288(nanocollimators). The beam generator 200 also comprises an acceleratingaperture 220 which extends through the semiconductor material and sharesa longitudinal axis with the collimating apertures 286 and 288. Thediameter of the accelerating aperture 220 is greater than the diameterof each of the collimating apertures 286 and 288. Typically, thediameter of the accelerating aperture might be around 50nm and thenanocollimator apertures of about 30nm. Particles will be emitted fromthe nanotip 214 if a sufficient voltage difference exists between thetip and the collimating aperture 286. These particles will beaccelerated and focused into an almost parallel beam if the voltagedifference across the semiconductor is sufficiently large enough. (Thearrow 290 shows the electron beam direction in both FIGS. 7 a and 7 b).Typically for an 0.5μm silicon thin waver, or film, the voltage acrossthe semiconductor might be around 300 volts and this will generate auniform field along the hole of 600MV/m. A longer nanocolumn is possibleif it is made in two stages as shown in FIG. 7 b. Here there are twolayers separated by a conducting film 213. The bottom layer 215 isconducting and can be made from metal or preferably very low resistivitydoped silicon. If the two metal films 213 and 215, are at earthpotential then the whole bottom column 285 is at earth potential. Thenanoaperture 286 performs the same function as in the device shown inFIG. 7 a but the aperture 288 which can be several microns from thenanotip is able to reduce scattering whilst further lowering the (phasespace) emittance of the electron beam. The hole in this lower column 285is fabricated at the same time as that of the upper acceleratingsection. Its sole function is to support the nanoaperture 288 concentricwith the hole in the semiconductor. A narrow electron beam, which islimited in diameter to the aperture size 288 then passes to theelectrostatic focussing elements of the microscope as shown in FIG. 8.

A complete particle beam generator system for use as a microscope isshown in FIG. 8 with the accelerating aperture 220 and the nanotip 214being the source of electrons. The narrow beam of electrons 222 passesfrom the nanocolumn 286/288 and through a concentric einzel lens asshown. This lens is a simple three-element arrangement which ismanufactured from conducting and insulating layers, 292 and 294,respectfully through which an aperture is manufactured. Multiple elementlenses, containing five or more electrodes, are also possible to reduceaberrations as previously mentioned for other embodiments. The insidediameter (aperture of the lens) and spacing of the electrodes is chosento give minimum aberrations and hence the smallest beam spot. Typicaldimensions for the lens are about 2μm for the inside diameter and eachlayer being about 1μm thick. Manufacture of the einzel lens issimplified if it is made from a single thin waver of three distinctlayers. Using silicon at different doping concentrations can produce aconducting layer 292 and an insulating layer 294. For a simple 3 elementlens the outer two conducting electrodes are at earth potential and thecentral one is at the correct voltage to give a focus at the desireddistance from the end of the assembly 296. This whole assembly forms thebody of the microscope and when this is fabricated at the edge of astepped assembly as previously mentioned in another embodiment the beamgenerator is essentially a single chip apart from the nanotip. Howeverthis nanotip is at the end of a cantilever so that it can be positionedon the centre of the nanocolumn entrance aperture and can thus beintegrated into the nanochip to make a complete focussed electron (ion)beam machine, namely a ‘Microscope on a Chip’. Note that the resistivefilm from which the microscope body is made can have many holes in it sothat they can all be accessed by moving the nanoprobe to any entranceaperture.

In the previous embodiment an accelerating nanocolumn is constructedfrom a multilayer structure of alternate metal (conducting) andinsulating layers through which is a hole of diameter of less then 100nm is fabricated and is the channel down which the electrons pass. Byapplying voltages to the conducting electrodes in this assembly it ispossible to produce a high electric field along the evacuated aperturein the column. This embodiment is a simpler method of producingnanocolumns or accelerators which have the same effect as the previousassembly. Furthermore this new device is simpler to manufacture and canaccommodate the inclusion of restricting (anti-scatter) collimators atboth ends of the column. The method is to manufacture the acceleratorfrom a single sheet of high resistivity material through which holes areproduced using microfabrication techniques. The favoured material,though not the only possibility is single crystal doped silicon as usedfor the manufacture of microchips. The doping will normally be n-type(though p-type is possible) and the doping density should be such thatthe resistivity is in the range from 1 kΩm-cm to 100 MΩm-cm but notexclusively. A voltage applied across a thin film of such a materialwill ensure that there is a uniform electric field along any straighthole through the resistive material. The hole is made normal to theparallel sides of the thin wafer or film, which is the body of theaccelerator and can be loosely termed a nanocolumn, in line with theprevious terminology for a column constructed from a multilayer ofalternate insulating and conducting thin films. (Nanocolumn is usedbecause the aperture through the film is in the nanometre size range.)In this circumstance the electric field is along the (evacuated) holeand it can thus accelerates electrons injected into the hole. A nanotip,which can be positioned above a hole of typical aperture 50 nm and at adistance of around 30 nm, will field emit electrons if the voltage onthe tip exceeds that of the surface by about 10 volts. Both surfaces ofthe semiconductor are covered with a thin metallic film through whichholes are manufactured concentric with the hole in the semiconductor.The diameter of the holes in the metallic film are smaller than that inthe semiconductor so that these apertures act as anti-scattercollimators and can also be used to reduce the electron beam emittance.

The operation of these nanocolumns in focussed electron and beam devicesis as follows. A negative voltage is applied to the metallic layernearest to the nanoprobe and larger negative voltage is applied to thenanotip. The metallic layer on the other semiconductor surface is atearth potential. By choosing these voltages correctly electrons emittedfrom the tip can be focussed and accelerated down the hole in thenanocolumn. An almost parallel beam of electrons with diameters lessthan 50 nm can be produced.

For the best performance, the diameter of a collimator aperture needs tobe less than 100 nm and the thickness of the silicon larger than 0.5 μm.If this arrangement is to be effective, it is essential that the devicecontains collimators to both reduce the scattering from the walls (ofthe nanoscale hole) and to reduce the total emittance of the beam. Thelatter can be extremely important since the total emittance of the beamis proportional to the final beam spot size. Thus a large decrease inemittance brought about by using carefully chosen collimators can leadto a significant reduction in the final beam spot size. Two methods areavailable for producing collimators at the nanoscale. In the firstmethod the nanoscale column has a conical hole in it with the smallerdiameter hole closest to the nanotip electron source. In this wayscattering of the electron beam from the inside walls of the hole can belargely eliminated. A conical shape can be replaced by a form in whichthe aperture of the hole is reduced more abruptly at the position wherecollimation is required. These collimators can be formed at both ends ofthe tube if needed. In another scheme, a thin metal covering layer ateither or both end(s) of the hole is ion etched to produce a collimator.This can be done by dry etching techniques or using a focussed ion beam(FIB) milling machine.

General arrangements are shown in FIG. 9, with the electron source beinga nanotip 314 at the entrance to the first nanoscale section of thedevice with the beam direction 390 being marked. FIG. 9 a shows asection of material 301 of micron thickness through which is fabricateda 50 nm (typical size) circular hole by dry etching techniques. Thewalls of this hole can be made parallel if the etching is carefullycontrolled. The whole microscope column or assembly can be made withaccelerating sections and non-accelerating sections as described in aprevious embodiment. One method of fabricating these apertures 386 and388 is as follows. During the production of the hole, registrationfeatures 398 are produced on the surface to delineate the apertureposition. The surface is then coated with a nanometer thick gold layerby vacuum deposition techniques (atomic deposition from a source) and a2–3 nm (typical) thick gold foil, 312 and 313 is placed over theaperture on top of this first layer. (If this is done in cleanconditions the gold foil will bond to the vacuum deposited gold layer onthe silicon.) It is then possible to produce apertures, 386 and 288 inthis metal foil by ion beam drilling or dry etching. (For this to bepossible it is important that the registration remains visible after thegold layer is applied.) FIG. 9 b shows an alternative way of producingan aperture particularly at the entrance to the accelerating section. Inthis method the hole is tapered into a conical shape as shown. Thistapering can be produced by carefully controlling the dry etchingprocess. The top conducting layer 399 is then made by depositing a metalon the surface using standard vacuum deposition methods. A furtheraperture made by the previous method can be placed below this assemblyas shown in the central diagram. However it is also possible to producea collimator at this position by placing a second wafer with a taperedhole in it below the one shown in the central diagram. This thenreplaces the aperture made from thin film metal (gold). It can be madein a separate thin wafer (of silicon) which is positioned so that theholes are concentric or the whole assembly can be fabricated in onepiece. Thus the system now consists effectively of two wafers withconical holes with both wafers vacuum coated on their flat sides withmetallic films. It is also possible to produce a collimator from theintrinsic material of the wafer not necessarily in the form of a taperas is shown in FIG. 9 c. Collimators can be manufactured at either orboth ends of the assembly or assemblies (wafers). These can be stackedto minimise scattering and/or reduce the phase space emittance of thebeam.

1. A particle beam generator, comprising an extracting plate, having anextracting aperture, disposed adjacent a particle source and operable toextract particles from such a source into the extracting aperture toform a particle beam, particle accelerating means operable to acceleratethe extracted particles to increase the energy of the beam, andcollimating means operable to collimate the particle beam, characterizedin that said extracting plate is disposed sufficiently close to saidparticle source such that this proximity, in combination with theprovision of an electric field applied to either side of said extractingaperture to provide a focusing effect on the particle beam passingthrough the extracting aperture, together inhibit lateral expansion ofsaid particle beam such that it has a diameter of less than 100 nm.
 2. Aparticle beam generator, as claimed in claim 1, further comprisingfocussing means operable to provide, from the laterally inhibitedparticle beam, a focussed particle beam having a diameter less than 1nm.
 3. A particle beam generator as claimed in claim 1, wherein thediameter of the extracting aperture is substantially between 5 nm and500 nm.
 4. A particle beam generator as claimed in claim 3, wherein thediameter of the extracting aperture is substantially between 5 nm and100 nm.
 5. A particle beam generator as claimed in claim 1, wherein theparticle accelerating means comprises a plurality of accelerator platesarranged in a stack and electrically isolated from each other, eachaccelerator plate having an aperture arranged to share a commonlongitudinal axis with the extracting aperture to form an extendedaccelerating aperture along which the extracted particles areaccelerated on application of a voltage between the extractor plate anda first accelerator plate and between each pair of successive adjacentaccelerator plates arranged in the column thereafter.
 6. A particle beamgenerator as claimed in claim 1, wherein the extracting plate is a firstconductor which is separated from a second conductor by at least one ofa resistive and insulator material, and the accelerating means comprisesan accelerating aperture which extends from the extractor aperturethrough the at least one of the resistive and insulator material andthrough the second conductor, wherein the extracted particles areaccelerated on application of a differential voltage between the firstand second conductors.
 7. A particle beam generator as claimed in claim6, wherein the resistance of the at least one of the resistive andinsulator materials is substantially between 1 kΩm-cm and infinity.
 8. Aparticle beam generator as claimed in claim 5, wherein the diameter theaccelerating aperture is substantially between 10 nm and 100 μm.
 9. Aparticle beam generator as claimed in claim 1, wherein the collimatingmeans is integrally formed with the accelerating means.
 10. A particlebeam generator as claimed in claim 9, wherein the accelerating meansincludes an accelerating aperture through which the particle beam passesto thereby create an accelerated beam, and wherein the collimating meanscomprises a conical configuration of said accelerating aperture, theconical accelerating aperture having a diameter increases in thedirection of the accelerated beam.
 11. A particle beam generator asclaimed in claim 10, wherein the collimating means comprises at leastone aperture having a lesser diameter relative to the acceleratingaperture and is disposed on the longitudinal axis thereof.
 12. Aparticle beam generator as claimed in claim 1, comprising a particlesource integrated therewith.
 13. A particle beam generator as claimed inclaim 12, wherein the particle source is a field emission source.
 14. Aparticle beam generator as claimed in claim 1 adapted for use with anelectron particle source.
 15. A particle beam generator as claimed inclaim 1 adapted for use with an ion particle source.
 16. A near fieldmicroscope comprising a particle beam generator as claimed in claim 1.17. A microchip comprising a particle beam generator as claimed in claim1.